An electrophotographic image forming apparatus (e.g. printer) includes a fixing apparatus that fuses a toner image formed on a paper sheet so as to fix the toner image on the paper sheet. Known as an example of the fixing apparatus is a fixing apparatus which includes a pair of rollers, a fixing roller and a pressure roller, as shown in Patent documents 1 through 4 below.
The fixing roller is a roller member that has (a) a hollow shaft which is made of metal such as aluminum, (b) an elastic layer which is formed on the surface of the hollow shaft, and (c) a halogen lamp which is disposed as a heat source inside the shaft. A temperature control device controls a temperature of the fixing roller by performing on-off control of the halogen lamp in accordance with a signal outputted from a temperature sensor which is provided on the surface of the fixing roller.
Apart from the aforementioned halogen lamp, an external heating member which is pressed against the outer surface of the fixing roller is known as means for heating the fixing roller. Specific examples of the external heating member include an external heating roller which is disclosed in Patent document 1 and an external heating belt which is disclosed in Patent documents 2 through 4. Since the external heating member directly contacts the outer surface of the fixing roller, the external heating member can heat the outer surface of the fixing roller more quickly than the aforementioned halogen lamp.
The pressure roller is a roller member which has a shaft and a heat-resistant elastic layer provided as a covering layer around the shaft. The heat-resistant elastic layer is made of a silicone rubber, for example. The pressure roller is pressed against the outer surface of the fixing roller, which causes elastic deformation of the elastic layer of the pressure roller. This provides a nip region between the fixing roller and the pressure roller.
In the above arrangement, a fixing apparatus is such that a paper sheet having unfixed toner image formed thereon is caught in the nip region between the fixing roller and the pressure roller and transported by rotation of the fixing roller and the pressure roller, and then the toner image formed on the paper sheet is fused by heat given off from the outer surface of the fixing roller so that the toner image can be fixed on the paper sheet.
In such a fixing apparatus, it is known that offset phenomenon such as cold offset and hot offset occurs when a temperature on the outer surface of the fixing roller falls outside an appropriate temperature range. The cold offset is a phenomenon in which due to lack of the amount of heat transferred to a paper sheet, a part of insufficiently melted toner sticks to the fixing roller. The hot offset is a phenomenon in which due to overheating of a toner on a paper sheet, a cohesive force of the toner decreases, and a part of the toner on the paper sheet sticks to the fixing roller.
Thus, in order to prevent such cold offset and hot offset, it is very important for the fixing apparatus to control a temperature of the fixing roller so that a temperature on the surface of the fixing roller is within the appropriate temperature range during sheet passing.
Patent document 1:
Japanese Unexamined Patent Publication No. 038802/1999 (Tokukaihei 11-038802; published on Feb. 12, 1999)
Patent document 2:
Japanese Unexamined Patent Publication No. 189427/2005 (Tokukai 2005-189427; published on Jul. 14, 2005)
Patent document 3:
Japanese Unexamined Patent Publication No. 292714/2005 (Tokukai 2005-292714; published on Oct. 20, 2005)
Patent document 4:
Japanese Unexamined Patent Publication No. 017031/1977 (Tokukaisho 52-017031; published on Feb. 8, 1977)
The appropriate temperature range of the outer surface of the fixing roller varies depending upon a paper transport speed of an image forming apparatus into which the fixing apparatus is installed. More specifically, the appropriate temperature range tends to shift to higher temperatures with increase of the paper transport speed (process speed) and shifts to lower temperatures with decrease of the paper transport speed. The reasons for the tendency are as follows. In cases where the paper transport speed is high, a time for the contact between the paper sheet and the outer surface of the fixing roller is short. Hence, sufficient heat is not transmitted from the outer surface of the fixing roller to the paper sheet unless a temperature on the outer surface of the fixing roller is relatively high. On the other hand, in cases where the paper transport speed is low, the contact time is long. Hence, excessive heat is transferred from the outer surface of the fixing roller to a paper sheet unless the temperature on the outer surface of the fixing roller is suppressed.
The so-called 4-cycle electrophotographic image forming apparatus is usually designed to have a substantially identical paper transport speed in (a) cases where a color image is formed on a paper sheet and (b) cases where a monochrome image is formed thereon (It is to be noted that an interval between the sheets of paper transported is different in the color image formation and monochrome image formation, and the number of sheets processed per unit time are therefore larger in the monochrome image formation.). Note that the 4-cycle electrophotographic image forming apparatus has a scheme in which one visible image forming unit forms toner images of four colors CMYK and has the toner images overlapped so as to form a color image.
In a fixing apparatus included in the 4-cycle electrophotographic image forming apparatus, when the appropriate temperature range (non-offset range) to fix color images is compared with the non-offset range to fix monochrome images, overlap range, i.e. common non-offset range exists between both of the non-offset ranges, and the common non-offset range is wide, as illustrated in FIG. 9. Therefore, by setting a control value (setting value) of a temperature of the surface of the fixing roller so as to fall within the common non-offset range, it becomes easy to perform temperature control such that a temperature of the outer surface of the fixing roller falls within an appropriate temperature range both in the case of color image fixing and the case of monochrome image fixing. This makes it possible to easily avoid the occurrence of the offset phenomena.
However, as to a recent four drum tandem engine image forming apparatus, i.e. image forming apparatus in which four visible image forming units corresponding to the four colors CMYK are disposed, a demand for designing to have a higher paper transport speed in forming a monochrome image on a paper sheet than in forming a monochrome image thereon has been increasing. According to this demand, paper transport speeds are greatly different between in monochrome image forming mode and in color image forming mode. This is because the image forming apparatus designed as above can increase the number of paper sheets processed for monochrome image without degrading image quality of a monochrome image formed.
As described above, in an image forming apparatus which is designed to increase the number of sheets processed for monochrome image, the non-offset range to fix color image and the non-offset range to fix monochrome image have a very narrow common non-offset range, as illustrated in FIG. 10. Thus, in cases where the common non-offset range is very narrow, it is difficult to perform temperature control for a surface temperature of the fixing roller falling within the common non-offset range even when a control value (setting value) for a surface temperature of the fixing roller is set so as to fall within the common non-offset range. In this case, the cold offset problem and hot offset problem are likely to occur.
Additionally, in paper passing operation following a standby state in the fixing apparatus, it is necessary to set the surface temperature of the fixing roller to a temperature at which a paper sheet can be passed in both the monochrome mode and the color mode. Thus, it is necessary to perform temperature control with respect to the fixing roller in such a manner a surface temperature of the fixing roller in a standby state is controlled to a temperature between a fixable lower limit temperature in the monochrome mode and a fixable upper limit temperature in the color mode so that switching between the monochrome mode and the color mode can be performed at a paper passing operation following the standby state.
Furthermore, undershoot or overshoot of a surface temperature of the fixing roller immediately after the completion of a warm-up of the fixing apparatus, causes the surface temperature of the fixing roller to fall outside the common non-offset range. When the fixing apparatus enters into the paper passing operation from the above-mentioned state, cold offset or hot offset is likely to occur in an early stage of the paper passing operation.